Mechanochemistry of Spiropyran Under Internal Stresses of a Glassy Polymer

Mechanophores are powerful molecular tools used in polymers to track bond rupture and characterize mechanical damage. The majority of mechanophores are known to respond to external stresses and in this study we report the first precedent of a mechanochemical response to internal, residual stresses that accumulate during polymer vitrification. While internal stress is intrinsic to polymers that can form solids, we demonstrate that it can dramatically affect the mechanochemistry of spyropyran probes and alter their intramolecular isomerization barriers by up to 70 kJ·mol-1. This new behavior of spiropyrans enables their application for analysis of internal stresses and their mechanochemical characterization on the molecular level. Spectroscopy and imaging based on spiropyran mechanochemistry showed high topological sensitivity and allowed to discern different levels of internal stress in various locations along the polymer chain. The nature of the developed technique allows for wide field imaging of stress heterogeneities in polymer samples of irregular shapes and dimensions, making it feasible to directly observe molecular level manifestations of mechanical stresses that accompany the formation of a vast number of solid polymer.